CARBONIFEROUS AGE. 29 



secwriformis. Hall, show traces of the broad striated area of that genus. A. fiahella, 

 Conrad, from the Hamilton group, has also been found to be a true Pterinia} From 

 aU that is therefore known in regard to the affinities of these extinct shells, we may 

 safely infer that probably all of our Silurian and Devonian species, usually referred 

 to Avicula, especially those of the Hamilton and Chemung groups, wUl be found to 

 possess the characters of Pterinia, or of undescribed genera. 



It is a fact worthy of note, that while the existing genera of the family Pteriidce 

 or Aviculidce, form a group at once so natural and distinctly defined that Concholo- 

 gists meet with little difficulty in deciding what particular forms it should include, 

 the boundaries of the family were not always so sharply marked. For, when we 

 undertake to classify the numerous extinct genera that were introduced, lived out 

 their term, and passed out of existence at various periods during the immense 

 interval of time between the first introduction of this type of life and the present 

 epoch, the case is very difierent ; since we find amongst the vast numbers of fossil 

 species, types presenting various intermediate gradations between the modern 

 representatives of this and some of the allied families. For instance, no Concholo- 

 gist could be for a moment in doubt whether any particular species or genus of our 

 existing moUusks belongs more properly to the Aviculidce, or to the Pectlnidce. Yet 

 in tracing these two families, by their fossil shells, back into the distant past, we 

 meet with various types presenting such an assemblage of characters as to often 

 render their proper distribution more difficult ; especially since we have only the 

 light of analogy to guide us in our conclusions respecting the structure of the 

 softer parts of these extinct forms. Some of these peculiar species were formerly 

 referred by many Palaeontologists to the genus Pecten, and by others to Avicula ; 

 and even now, since the genus Avicidopecten has been established for their recep- 

 tion, authors are by no means agreed whether this genus should be classed with 

 the Pectinidce or the Avicididce. 



Again, no one having even a small amount of conchological knowledge, need be 

 at a loss in deciding to which of the two families, Arcidce or Aviculidce, any of our 

 existing species of bivalves belongs. Yet in passing from group to group of the 

 Arcidce, from the recent typical examples through some of the other modern forms, 

 and thence through various extinct t}'pes, it will be observed that the hinge plates, 

 or denticles, become more and more oblique, until in some of the Palseozoic genera, 

 such as Cyrtodonta, Vantixemia, Dolabra, Sec, only a few obscure divisions are to 

 be seen at the remote extremities of the hinge, ranging nearly or quite parallel to 

 the cardinal margin, as in BaJcevellia, Pterinia, and other genera apparently belonging 

 to the Avicididce. In addition to this, in many of the extinct groups of AvicuUdcs, 

 such, for instance, as GrTjpliorhynchus, Myalina, Bahevellia, &c., there is as well a 

 developed cardinal area, as we generally see m the ArcidcB ; while this area in 



* See remarks on the family Aviculidse, by F. B. Meek, Am. Jour. Sci. and Arts (2cl ser.) Tol- 

 XXXVII, March, 1864, 46. 



